Given the task definition and input, reply with output. Given a context passage, generate a question from the passage such that its answer is shortest continous span from the passage.

Several properties of the atmosphere change with altitude, but the composition of the natural gases does not. The proportions of gases in the atmosphere are everywhere the same, with one exception. At about 20 km to 40 km above the surface, there is a greater concentration of ozone molecules than in other portions of the atmosphere. This is called the ozone layer. Nitrogen and oxygen together make up 99% of the planets atmosphere. Nitrogen makes up the bulk of the atmosphere, but is not involved in geological or biological processes in its gaseous form. Nitrogen fixing is described in the chapter Life on Earth. Oxygen is extremely important because it is needed by animals for respiration. The rest of the gases are minor components but sometimes are very important (Figure 1.1). Nitrogen and oxygen make up 99% of the atmosphere; carbon dioxide is a very important minor component. Humidity is the amount of water vapor in the air. Humidity varies from place to place and season to season. This fact is obvious if you compare a summer day in Atlanta, Georgia, where humidity is high, with a winter day in Phoenix, Arizona, where humidity is low. When the air is very humid, it feels heavy or sticky. Dry air usually feels more comfortable. When humidity is high, water vapor makes up only about 4% of the atmosphere. Where around the globe is mean atmospheric water vapor higher and where is it lower (Figure 1.2)? Why? Higher humidity is found around the equatorial regions because air temperatures are higher and warm air can hold more moisture than cooler air. Of course, humidity is lower near the polar regions because air temperature is lower. Remember that greenhouse gases trap heat in the atmosphere. Important natural greenhouse gases include carbon dioxide, methane, water vapor, and ozone. CFCs and some other man-made compounds are also greenhouse gases. Some of what is in the atmosphere is not gas. Particles of dust, soil, fecal matter, metals, salt, smoke, ash, and other solids make up a small percentage of the atmosphere and are called particulates. Particles provide starting points (or nuclei) for water vapor to condense on and form raindrops. Some particles are pollutants. Click image to the left or use the URL below. URL:

humidity is how much of this is in the air.